The goal is to carry a change of clothes and the MacBook Air (1.7 cm high, 29.95 cm wide, 19.2 cm deep and 1.06 kg) on a 14km commute. I've used Ortlieb panniers (on a Tubus rack) but I don't like the way my heels can hit them, they way they stick out on the sides or the way they add drag. Nicely made though! Also I don't want all the weight on one side and I don't want to carry two of them.
A backpack is way too sweaty.
The best rack top solution so far is a Rixen & Kaul Rackpack 1 mounted on a matching Rixen & Kaul Freerack (sorry, can't find a link for that one). A MacBook Air 11" (bare - no sleeve), a pair of jeans and a T-shirt are a tight fit. Other stuff goes in the side and it's workable. But I can see that it isn't a great solution because the laptop has to be placed in diagonally with the thin edge sticking up. Over time the sharp edge of the laptop is definitely going to wear through the bag, and any crash is going to cause a lot of damage to the laptop.
I have also tried a Deuter 10l rack top pack - it is also too small, not quite as good as the Rixen & Kaul and the velcro attachment is much slower and less secure.
I've got a lot of nice things to say about Rixen & Kaul. It is a very well made bag and when I had a problem with the clip they sorted it out right away. This bag is just a little bit too small to be perfect. The click fix system is great. I have thought of removing the click fix mount and assign it to some other bag but it's a last resort and chances are my home-made solution is not going to be as nice as the ideal bag I'm wanting.